(1) Field of the Invention
The present invention relates to a novel composite one-component type magnetic developer. More particularly, the present invention relates to a composite one-component type magnetic developer which can form on a plain paper a clear transferred image having a high density.
(2) Description of the Prior Art
One-component type magnetic developers consisting of particles comprising a finely divided magnetic material dispersed in a binder medium have been used widely in the field of electrophotographic reproduction because any particular magnetic carrier need not be incorporated in these developers at the step of magnetic brush development. In a one-component type magnetic developer of the type used most frequently in the art, the surfaces of the developer particles are rendered electrically conductive so as to prevent the edge effect and fogging. However, if an electrically conductive magnetic developer of this type is transferred onto a copy paper having a relatively low electric resistance such as a plain paper, only an obscure image having broadened contours is obtained. This is a fatal defect of developers of this type.
It has already been known that a one-component type magnetic developer having a high electric resistance is used as the one-component type magnetic developer. For example, the specification of U.S. Pat. No. 3,645,770 discloses a process in which a magnetic brush of a one-component type magnetic developer having a high electric resistance is charged by corona discharge and an electrostatic latent image is developed by the charged magnetic brush of the magnetic developer. However, this process is still insufficient in various points. For example, it is very difficult to charge the magnetic brush uniformly, and a special charging mechanism should be arranged in the development apparatus zone and the development apparatus becomes complicated. Furthermore, the charging mechanism is readily contaminated with the developer and troubles are often caused by contamination of the charging mechanism.
Recently, there have been proposed a process in which an electrically non-conductive developer is charged by frictional contact with the surface of a support carrying thereon an electrostatic latent image to visualize the latent image by the thus charged developer (see Japanese Patent Application Laid-Open Specification No. 62638/75) and a process in which development is accomplished by dielectric polarization of an electrically non-conductive developer (see Japanese Patent Application Laid-Open Specification No. 133028/76). These processes, however, are still insufficient.
In the former process, the development conditions should be strictly controlled. More specifically, if the development conditions are not strictly controlled, fogging is readily caused in non-image areas (especially conspicuous when the degree of contact between the surface of a photosensitive material and the top ends of spikes of magnetic toner particles is high), and magnetic toner particles are fixed to a development sleeve and blocking of the toner particles is readily caused. Occurrence of these troubles is especially serious when the copying operation is performed continuously.
In the latter process, the trouble of fogging is not serious, but since a visible image is formed by utilization of a dielectric polarizing effect induced in the magnetic toner to an electrostatic latent image, a low-potential part of the latent image area is not effectively developed and therefore, the obtained copy has a hard tone and it is impossible to reproduce a half-tone in prints.
Moreover, images of prints obtained according to these conventional processes are poor in sharpness and according to these conventional processes, it is very difficult to form images having a sufficiently high density.
As another one-component type magnetic developer, there has been proposed a composite magnetic developer consisting of a homogeneous mixture comprising a magnetic developer having a high electric resistance and an electrically conductive magnetic developer. Furthermore, a composite developer consisting of a homogeneous mixture comprising a magnetic developer and a non-magnetic developer. However, these known composite developers are defective in that the composition is gradually changed while the copying operation is conducted continuously and the density of transferred images is considerably reduced.